Aromatic hydrocarbons, particularly benzene, toluene, ethylbenzene and xylenes, are important commodity chemicals in the petrochemical industry. Currently, aromatics are mostly frequently produced from petroleum-based feedstocks by a variety of processes, including catalytic reforming and catalytic cracking. However, as the world supplies of petroleum feedstocks decrease, there is a growing need to find alternative sources of aromatic hydrocarbons.
One possible alternative source of aromatic hydrocarbons is methane, which is the major constituent of natural gas and biogas. World reserves of natural gas are constantly being upgraded and more natural gas is currently being discovered than oil. Because of the problems associated with transportation of large volumes of natural gas, most of the natural gas produced along with oil, particularly at remote places, is flared and wasted. Hence the conversion of alkanes contained in natural gas directly to higher hydrocarbons, such as aromatics, is a particularly attractive method of upgrading natural gas, providing the attendant technical difficulties can be overcome.
A large majority of the processes for converting methane to liquid hydrocarbons involve first conversion of the methane to synthesis gas, a blend of H2 and CO. Production of synthesis gas is capital and energy intensive; therefore routes that do not require synthesis gas generation are preferred.
A number of alternative processes have been proposed for directly converting methane to higher hydrocarbons. One such process involves catalytic oxidative coupling of methane to olefins followed by the catalytic conversion of the olefins to liquid hydrocarbons, including aromatic hydrocarbons. For example, U.S. Pat. No. 5,336,825 discloses a two-step process for the oxidative conversion of methane to gasoline range hydrocarbons comprising aromatic hydrocarbons. In the first step, methane is converted to ethylene and minor amounts of C3 and C4 olefins in the presence of free oxygen using a rare earth metal promoted alkaline earth metal oxide catalyst at a temperature between 500° C. and 1000° C. The ethylene and higher olefins formed in the first step are then converted to gasoline range liquid hydrocarbons over an acidic solid catalyst containing a high silica pentasil zeolite.
However, these oxidative coupling methods suffer from the problems that they involve highly exothermic and potentially hazardous methane combustion reactions and that they generate large quantities of environmentally sensitive carbon oxides.
Dehydroaromatization of methane via high-temperature reductive coupling has also been proposed as a route for upgrading methane into higher hydrocarbons, particularly ethylene, benzene and naphthalene. Thus, for example, U.S. Pat. No. 4,727,206 discloses a process for producing liquids rich in aromatic hydrocarbons by contacting methane at a temperature between 600° C. and 800° C. in the absence of oxygen with a catalyst composition comprising an aluminosilicate having a silica to alumina molar ratio of at least 5:1, said aluminosilicate being loaded with (i) gallium or a compound thereof and (ii) a metal or a compound thereof from Group VIIB of the Periodic Table.
In addition, U.S. Pat. No. 5,026,937 discloses a process for the aromatization of methane which comprises the steps of passing a feed stream, which comprises over 0.5 mole percent hydrogen and 50 mole percent methane, into a reaction zone having at least one bed of solid catalyst comprising ZSM-5 and phosphorous-containing alumina at conversion conditions which include a temperature of 550° C. to 750° C., a pressure less than 10 atmospheres absolute (1000 kPaa) and a gas hourly space velocity of 400 to 7,500 hr−1. The product effluent is said to include methane, hydrogen, at least 3 mole % C2 hydrocarbons and at least 5 mole % C6-C8 aromatic hydrocarbons. After condensation to remove the C4+ fraction, cryogenic techniques are proposed to separate the hydrogen and light hydrocarbons (methane, ethane, ethylene, etc.) in the product effluent.
U.S. Pat. Nos. 6,239,057 and 6,426,442 disclose a process for producing higher carbon number hydrocarbons, e.g., benzene, from low carbon number hydrocarbons, such as methane, by contacting the latter with a catalyst comprising a porous support, such as ZSM-5, which has dispersed thereon rhenium and a promoter metal such as iron, cobalt, vanadium, manganese, molybdenum, tungsten or a mixture thereof. The addition of CO or CO2 to the feed is said to increase the yield of benzene and the stability of the catalyst.
However, existing proposals for the dehydroaromatization of methane frequently have low selectivity to aromatics and may require co-feeding of expensive additives to improve the aromatics selectivity. Moreover, any reductive coupling process generates large quantities of hydrogen and so, for economic viability, requires a route for effective utilization of the hydrogen by-product. Since natural gas fields are frequently at remote locations, effective hydrogen utilization can present a substantial challenge.
Another problem involved in the use reductive coupling to upgrade methane to higher hydrocarbons is that significant heat must be supplied to reaction. Thus not only is the process is highly endothermic, but also the reaction is thermodynamically limited. Thus the cooling effect caused by the reaction lowers the reaction temperature sufficiently to greatly reduce the reaction rate and total thermodynamic conversion if make-up heat is not provided in some manner. Various methods have been proposed for supplying heat to the aromatization of methane, but to date none of the proposed methods have proved entirely satisfactory.
For example, one known method of providing the heat of reaction to a methane aromatization process is the use of a heat-exchange fluid flowing through the reaction zone, which provides indirect heat to the catalyst in the reaction zone. However, this method of heat exchange tends to be inefficient and causes disruption of catalyst flow in non-fixed bed reactors.
It is also known to supply heat to a reaction to a methane aromatization process by using more than one reaction zone in sequence, in combination with reheating the reactants between the reaction zones. In this interstage reheating, the reactor effluent from a first bed of catalyst is heated to the desired inlet temperature of a second, downstream bed of catalyst.
One method of interstage reheating includes the use of indirect heat exchange, in which the effluent from an upstream reaction zone is passed through a heat exchanger before being fed to a subsequent reaction zone. The high temperature fluid employed in this indirect heat exchange method may be high temperature steam, combustion gases, a high temperature process stream or any other readily available high temperature fluid. This method of interstage heating does not dilute the reactants but does impose some pressure drop in the system and can expose the reactants to undesirably high temperatures.
For example, Russian Patent No. 2,135,441 discloses a process for converting methane to heavier hydrocarbons, in which the methane is mixed with at least 5 wt % of a C3+ hydrocarbon, such as benzene, and then contacted with a catalyst comprising metallic platinum having a degree of oxidation greater than zero at a methane partial pressure of at least 0.05 MPa and a temperature of at least 440° C. The process is conducted in a multi-stage reactor system using interstage reheating by indirect heat exchange. Hydrogen generated in the process may be contacted with oxides of carbon to generate additional methane that, after removal of the co-produced water, can be added to the methane feed. The products of the methane conversion are a C2-C4 gaseous phase and a C5+ liquid phase but, according the Examples, there is little (less than 5 wt %) or no net increase in aromatic rings as compared with the feed.
Another method of interstage heating is the oxidative reheat method that involves the admixture of a controlled amount of oxygen into the reactants and the selective oxidation of hydrogen generated in the aromatization process. The oxidation is accomplished in the presence of a catalyst that selectively promotes the oxidation of hydrogen as compared to the destructive combustion or oxidation of the more valuable feed and product hydrocarbons. However, the reaction generates steam that can be detrimental to the aromatization catalyst and can react with methane to form hydrogen and carbon monoxide. Moreover, by using a second selective oxidation catalyst, this method suffers from added complexity and cost.
An alternative approach to supplying heat of reaction to the reductive coupling process makes use of the fact that the catalyst generates coke as the aromatization reaction proceeds. This coke gradually deactivates the catalyst and hence the catalyst must be repeatedly regenerated to remove the coke and reactivate the catalyst. The regeneration, which involves contacting the catalyst with an oxygen-containing gas, is highly exothermic and hence can be used as a source of sensible heat to the overall process. Such a process is disclosed in International Patent Publication No WO 03/000826, in which a dehydroaromatization catalyst is circulated between a reactor system and a regenerator system, where the catalyst is contacted with different regeneration gases, including O2, H2, and H2O, at different times to regenerate different portions of catalyst. The percentage of catalyst contacting each regeneration gas is controlled to maintain the reactor system and regeneration system under a heat balance regime. The reactor system includes a fluidized bed of catalyst in a riser reactor, and the regeneration system includes a second fluidized bed of catalyst maintained in a bubbling bed reactor.
However, processes that use the catalyst regeneration step to supply reaction heat suffer from the problem that the catalyst needs to be heated well above the target reaction temperature in the regeneration process, which leads to accelerated catalyst degradation and hence reduced catalyst life. Moreover, to maintain heat balance, the process requires a high selectivity to coke rather than to the desired aromatic products.
There is therefore a need for an improved process for supplying heat of reaction to the aromatization of methane.